Present image display systems include the ability to display a small auxiliary image in addition to a larger main image. This smaller image may be displayed within the boundaries of the larger main picture, in which case, such a system is termed a picture-in-picture (PIP) system, or the smaller image may be located outside (e.g. to the left or right side of the main image, in which case the system is termed a picture-outside-picture (POP) system. The main and auxiliary images may be derived from the same image source, such as a freeze frame PIP image of the main image, or may be derived from an independent source, such as a system in which one tuner tunes one video signal which is displayed as the main image, and a second tuner tunes a second video signal, independent of the first tuner, which is displayed as the inset image.
A PIP or POP system operates by storing compressed image data representing the auxiliary image as it occurs in the auxiliary video signal, and then substituting this compressed image data for the main image signal at the portion of the main image which is designated to display the auxiliary image. The system must supply an amount of memory sufficient to store the auxiliary image data from the time it occurs in its video signal to the time it is displayed in the main image. Known systems provide sufficient memory to hold either a frame or a field of auxiliary video data. Because memory is relatively expensive, it is desirable to minimize the amount of memory required. To decrease the amount of memory required, known PIP and POP systems subsample the auxiliary video signal, and store only a single field of subsampled auxiliary video data. A display method, complementary to the subsampling method, is used to display the PIP or POP image.
Known subsampling techniques, however, consist of straightforward `take one sample, discard N samples` repeated for each line in the auxiliary video signal. This undesirably decreases the horizontal resolution of the PIP or POP image, which, in turn, decreases the perceived quality of the displayed PIP or POP image. A subsampling method which can increase the horizontal resolution of a PIP or POP image, without increasing the amount of memory necessary to store the PIP or POP image data for later display with the main image is desirable.
Quincunx subsampling is one method to increase the perceived horizontal resolution of the subsampled auxiliary image without increasing the amount of memory required to temporarily store the subsampled auxiliary image data. A quincuncial pattern is defined, in terms of spatial sampling, as four samples arranged in a square and one sample in the middle of the square. Quincunx sampling involves subsampling one line of the auxiliary image at a first set of equally spaced horizontal locations, and subsampling the next adjacent horizontal line (i.e. in the next field) at a second set of horizontal locations halfway between the first set, and repeating this pattern. The top portion of FIG. 3, described in more detail below, illustrates this sampling technique where the "X"s represent subsampled samples, and the "+"s represent skipped samples. As can be seen, and as will be described below, the same number of samples are taken in each horizontal line, but the samples, taken all together, cover twice as many horizontal locations, thus increasing the perceived horizontal resolution of the subsampled auxiliary image.
Because spatially adjacent lines occur in temporally adjacent fields, quincunx sampling may be accomplished by sampling all the lines in one field of the auxiliary image at one set of horizontal locations, and sampling all the lines in the next adjacent field at a second set of horizontal locations halfway between the horizontal locations of the preceding field. This pattern is repeated. Thus, each line within a field is sampled identically to all the other lines in that same field, but the horizontal locations of subsampled samples in the lines in one field are halfway between the horizontal locations of the subsampled samples in lines in the adjacent fields.